Technique for growth of single crystal gallium garnet

ABSTRACT

Rare earth gallium garnet crystals evidencing low dislocation densities are obtained by the crystal pulling techniques utilizing an oxygen containing ambient wherein the partial pressure of oxygen ranges from about 3.8-19 millimeters of mercury.

United States Patent 91 Brandle, JrQet al.

[ 1 Mar. 27, 1973 TECHNIQUE FOR GROWTH OF SINGLE CRYSTAL GALLIUM GARNET Inventors: Charles David Brandle, Jr., Somerville; David Christopher Miller, Millington, both of NJ.

Assignee: Bell Telephone Laboratories, Incorporated, Murray Hill, Berkeley Heights, NJ.

Filed: Aug. 18, 1971 Appl. No.: 172,751

US. Cl. ..423/263, 423/593, 23/301 SP Int. Cl. .....C0lf 17/00 Field of Search ..423/263, 593; 23/20, 51 R,

Harrison, Research Vol. 12, 1959, pp. 395-403.

Primary ExaminerHerbert T. Carter Attorney-R. J. Guenther et al.

[ 57] ABSTRACT Rare earth gallium garnet crystals evidencing low dislocation densities are obtained by the crystal pulling techniques utilizing an oxygen containing ambient wherein the partial pressure of oxygen ranges from about 3.8-19 millimeters of mercury.

9 Claims, 3 Drawing Figures PATENTEDI-IARZ? I973 FIG 2 FIG. 3

6 PRECIPITATE DENSITY XIO 6 PRECIPITATE DENSITY XIO (PARTICLES/cc) (PARTICLES/cc) PRECIPITATE DENSITY XIO (PARTICLES/cc) STOICHIOMETRIC MELTS 0F GROWTH TIME (HRS) MELTS CONTAINING 4WT EXCESS m s 8 IO I2 I4 GROWTH TIME (HRS) MELTS CONTAINING 4WT /o EXCESS 6 IO I2 I 1 GROWTH TIME (HRS) TECHNIQUE FOR GROWTH OF SINGLE CRYSTAL GALLIUM GARNET FIELD OF THE INVENTION This invention relates to a technique for the growth of single crystal synthetic garnet. More particularly, the present invention relates to the growth of defect-free, rare earth gallium garnet.

DESCRIPTION OF THE PRIOR ART In recent years, there has been a birth of interest in a class of devices comprising a magnetic medium in which a reverse magnetized domain is nucleated by a nucleation field in excess of a nucleation threshold and in which reverse domains are propagated in response to a propagation field in excess of a propagation threshold less than the nucleation threshold. These devices, commonly referred to as magnetic domain devices, have gained wide-spread prominence in the electronics industry and are used extensively for information storage and propagation.

Typically, such devices include a known magnetic garnet structure bearing an epitaxial film of a magnetic garnet. In light of the rigid requirements for crystalline perfection of the epitaxial films so employed, workers in the art have continually sought to produce defectfree garnet substrates. Heretofore, it has been common practice to grow garnet substrates for such applications byconventional cyrstal pulling techniques, as, for example, the well-known Czochralski procedure. Studies of rare earth gallium garnet grown in accordance with such techniques have revealed that the grown materials evidence low dislocation densities. Nonetheless, small particle inclusions therein restrict the available surface area suitable for use in device applications.

SUMMARY OF THE INVENTION In accordance with the present invention, this prior art deficiency has been successfully obviated by modifying the growth procedure. This end has conveniently been attained by effecting rare earth gallium garnet growth in an oxidizing ambient wherein the partial pressure of oxygen is maintained within a critical prescribed range prior to and during growth of the desired crystal. Growth in the described manner has been found to result in the total elimination of the deleterious inclusions which have been attributed to the decomposition of the rare earth gallium garnet to yield gallium suboxide, rare earth sesquioxide, oxygen and a reduced rare earth gallium oxide.

BRIEF DESCRIPTION OF THE DRAWING The invention will be more fully understood by reference to the following detailed description taken in conjunction with the accompanying drawing wherein FIG. 1 is a graphical representation on coordinates of growth time versus inclusion density showing the pregrowth conditions required to eliminate inclusions for stochiometric melts of gadolinium gallium garnet utilizing varying concentrations of oxygen;

FIG. 2 is a graphical representation on coordinates of growth time versus inclusion density showing the pregrowth conditions required to eliminate inclusions for melts of gadolinium gallium garnet containing 4 weight percent excess gadolinium oxide utilizing varying concentrations of oxygen; and

FIG. 3 is a graphical representation on coordinates of growth time versus inclusion density showing the pregrowth conditions required to eliminate inclusions for melts of gadolinium gallium garnet containing 4 weight percent excess gallium oxide utilizing varying concentrations of oxygen.

DETAILED DESCRIPTION OF THE INVENTION The synthetic garnet materials here considered can be represented by the formula M Me Me 'O where M is one of the rare earth elements of atomic number between 62 and 71 or a mixture of these rare earth elements with each other, Me is gallium or scandium, Me is gallium and O is oxygen.

As noted above, the formula for the rare earth garnet of interest is M Me Me 'O so indicating a molecular ratio of 3 parts of rare earth oxide to 5 parts of gallium oxide or gallium and scandium oxides. The maintenance of this mol ratio, known as the stochiometric ratio, yields satisfactory crystalline growth in accordance with this invention. However, variations from the stochiometric ratio may be made without interfering with crystallization of the desired material. Thus, it has been found that excesses beyond stochiometry may be employed with respect to both gallium and the rare earth oxides, such excess varying up to 7 weight percent in each case. It has been determined that the use of excesses of either gallium or rare earth oxide beyond the noted limits results in the formation of a polycrystalline material which is of no value from a device standpoint.

In accordance with the inventive technique, studies have revealed that defect-free rare earth gallium garnet can be grown utilizing a growth ambient containing oxygen and maintaining the melt in the oxygen ambient for a prescribed period of time prior to growth for the purpose of effecting oxidation of precipitates present therein. The growth ambient may contain from 0.5 to 2.5 volume percent oxygen which corresponds with an oxygen partial pressure ranging from 3.8 to 19.0 millimeters of mercury. The melt comprising the constituent components of the desired crystal is maintained in the oxygen ambient for a time period ranging from 1.5 to 10 hours, the shorter time period corresponding with the higher oxygen partial pressure and the longer time period corresponding with the lower oxygen pressure. During this pregrowth stage of the process, the deleterious precipitates alluded to above are oxidized, so permitting the subsequent growth of inclusion-free material. It should be noted that failure to employ at least 0.5 volume percent oxygen for the required time period in this stage of the processing will result in the formation of precipitates and ultimately in an unacceptable level of inclusion density in the grown garnet. At the opposite end of the range, it has been found the use of more than 2.5 volume percent oxygen in the growth ambient leads to excessive crucible oxidation, iridium typically being employed for this purpose. Under such circumstances, the resultant iridium oxide would dissolve in the melt and result in increased interface instability,'strain and ultimately, dislocations in the crystal. An optimum oxygen partial pressure has been found to increase with 2 volume percent oxygen or 15.2 millimeters of mercury.

With reference now more particularly to the drawing, FIG. 1 is a graphical representation showing inclusion density as a function of growth time for stochiometric melts of gadolinium gallium garnet at varying oxygen partial pressures. A review of the data plotted in FIG. 1, as well as in FIGS. 2 and 3, reveals that a certain pregrowth period is required to effect complete oxidation of precipitates at various concentrations of oxygen. Thus, it will be noted that at 2 volume percent oxygen it takes approximately 1% hours to eliminate the precipitates, whereas at one-half volume percent oxygen a time period of approximately 7 hours is required to attain this end. FIGS. 2 and 3 present data in graphical form for gadolinium gallium garnet melts containing 4 weight percent excess of gadolinium oxide and gallium oxide, respectively. The results plotted in these figures indicates that the pregrowth time periods required to eliminate precipitates are essentially similar in each case.

Examples of the application of the present invention are set forth below. They are intended merely as illustrations and it is to be appreciated that the processes described may be varied by one skilled in the art without departing from the spirit and scope of the invention.

EXAMPLE 1 2 14.85 gramsof gadolinium oxide and 185.15 grams of gallium oxide, obtained from commercial sources, were weighed into an iridium crucible and heated to a temperature of approximately 1700C, the melting point of the mixture. Heat was effected by coupling the crucible with an RF induction heater. The crucible, together with its contents, was then permitted to attain a temperature of l750C at which point the charge was entirely liquid. Prior to the introduction of oxygen into the system, the melt was maintained in a neutral atmosphere, nitrogen, for one-half hour to insure a constant formation rate of the reduced compound. Next, oxygen was admitted to the system at a partial pressure of 15.2 millimeters of mercury and the melt maintained in this ambient at a temperature of 1750C for 2 hours. Then, a 20 mil platinum wire bearing a gadolinium gallium garnet seed crystal was inserted into the melt and the Czochralski pulling technique employed to grow gadolinium gallium garnet to 0.5 inch diameter at a growth rate of 0.2 inch per hour in the oxygen ambient containing 2 volume percent oxygen. A lengthwise center slice was then taken from the resultant grown crystal for determination of inclusion density. It was observed that the decomposition reaction normally occurring in the liquid garnet which results in inclusion formation in a neutral atmosphere throughout the entire length of the crystal does not occur in the oxidizing ambient and the inclusion density was found to be zero.

EXAMPLE 2 The procedure of Example 1 was repeated with the exception that the oxygen partial pressure was maintained at 0.5 percent by volume prior to and during growth, the pregrowth stage being conducted for 7 hours. It was observed that the inclusion density in the resultant grown crystal was zero.

EXAMPLE 3 The procedure of Example 1 was repeated with the exception that the oxygen partial pressure was maintained at 1 volume percent prior to and during growth, the pregrowth stage lasting for 5 hours. The inclusion density of the resultant crystal was found to be zero.

I EXAMPLE 4 The procedure of Example 1 was repeated with the exception that the melt contained 4 weight percent excess gadolinium oxide, the pregrowth stage lasting for 2 hours. The resultant grown crystal evidenced an inclu' sion density of zero.

EXAMPLE 5 The procedure of Example-2 was repeated with the exception that the melt contained 4 weight percent excess gadolinium oxide, the pregrowth stage lasting for 4 hours. The resultant grown crystal evidenced an inclusion density of zero.

EXAMPLE 6 The procedure of Example 3 was repeated with the exception that the melt contained 4 weight percent excess gadolinium oxide, the pregrowth stage lasting for 4 hours. The resultant grown crystal evidenced an inclusion density of zero.

EXAMPLE 7 The procedure of Example 1 was repeated ,with the exception that the melt contained 4 weight percent excess gallium oxide, the pregrowth stage lasting for 2.5 hours. The resultant grown crystal evidenced an inclusion density of zero.

EXAMPLE 8 The procedure of Example 2 was repeated with the exception that the melt contained 4 weight percent excess gallium oxide, the pregrowth stage lasting for 7 hours. The resultant grown crystal evidenced an inclusion density of zero.

EXAMPLE 9 The procedure of Example 3 was repeated with the exception that the melt contained 4 weight percent excess gallium oxide, the pregrowth'stage lasting for 5 hours. The resultant grown crystal evidenced an inclusion density of zero.

EXAMPLE 10 The procedure of Example 1 was repeated employing 70.59 grams of gadolinium oxide, 17.91 grams of scandium oxide and 36.50 grams of gallium oxide. The melt temperature was 1825C and it was elevated to l875C for 2 hours prior to growth. The resultant Gd Sc2Ga O12 crystal evidenced an inclusion density of zero.

What is claimed is:

1. Technique for the growth of synthetic garnet of the general formula M Me Me O where M is at least one element selected from the group consisting of the rare earth elements with atomic number between 62 and 71, inclusive, Me is selected from the group consisting of gallium and scandium, Me is gallium and O is oxygen, which comprises preparing a melt comprising the constituent components of said garnet in oxide.

form and exposing said melt to an oxygen containing ambient having a partial pressure ranging from 3.8 to 19.4 millimeters of mercury for a time period ranging from 1 to hours, the shorter time periods corresponding with the higher oxygen pressures and the converse, and subsequently effecting growth by crystal pulling techniques while maintaining the partial pressure of the oxygen containing ambient within the range of 3.8 to 19.4 millimeters of mercury.

2. Technique in accordance with claim 1 wherein the melt comprises a stochiometric mixture of the constituent components of said garnet.

3. Technique in accordance with claim 1 wherein the melt includes an excess of the constituent components of said garnet ranging up to 7 percent by weight.

4. Technique in accordance with claim 1 wherein said rare earth element is gadolinium.

5. Technique in accordance with claim 2 wherein Me is scandium.

6. Technique in accordance with claim 3 wherein said melt comprises an excess of gallium oxide.

7. Technique in accordance with claim 4 wherein Me is scandium.

8. Technique in accordance with claim 4 wherein said melt comprises an excess of gadolinium oxide.

9. Technique in accordance with claim 8 wherein the partial pressure of oxygen is maintained at 15.2 millimeters of mercury prior to and during growth. 

2. Technique in accordance with claim 1 wherein the melt comprises a stochiometric mixture of the constituent components of said garnet.
 3. Technique in accordance with claim 1 wherein the melt includes an excess of the constituent components of said garnet ranging up to 7 percent by weight.
 4. Technique in accordance with claim 1 wherein said rare earth element is gadolinium.
 5. Technique in accordance with claim 2 wherein Me is scandium.
 6. Technique in accordance with claim 3 wherein said melt comprises an excess of gallium oxide.
 7. Technique in accordance with claim 4 wherein Me is scandium.
 8. Technique in accordance with claim 4 wherein said melt comprises an excess of gadolinium oxide.
 9. Technique in accordance with claim 8 wherein the partial pressure of oxygen is maintained at 15.2 millimeters of mercury prior to and during growth. 